Support cross-chain

[AdamISZ]

At the moment only a ghost of this idea in the handshake/config (source_chain, destination_chain, currently only BTC supported).

This will obviously be a bit difficult depending on the other chain, will require a new back-end package, duplicating the effects of jmbitcoin, and will somehow support pricing.

Where it's possible at all it requires careful handling of blockchain timeouts (and requires CLTV/CSV capability in the scripting).

No plans to do this for now.